This invention relates to a device for use in attaching an electronic component to a substrate by a plurality of solder bump interconnections formed by reflowing preformed solder bumps. More particularly, this invention relates to a device that is assembled with the component and the substrate in preparation for forming the interconnections and is actuated at an appropriate time during solder reflow operations to stretch the molten solder bumps and thereby form interconnections having a desired hourglass configuration.
A common electronic component package comprises a component, such as a pad array carrier, mounted onto substrate, such as a printed circuit board, by a plurality of solder bump interconnections. The component has a generally planar inner face comprising terminal bond pads formed of solder wettable metal and disposed in a array or other suitable pattern. A solder bump is applied to each bond pad, for example, by reflowing preformed microballs of the solder alloy. Alternately, the bumps may be applied by plating or vapor depositing the solder alloy. The substrate includes a generally planar component attachment region having terminal bond pads disposed in a corresponding pattern. In preparation for attachment, the component is superimposed on the substrate region such that each bump rests upon a corresponding pad on the substrate. The assembly is heated to reflow the solder alloy onto the substrate pad and, upon cooling, form the interconnection. The resulting package features the component with the inner face spaced apart from the substrate region by the solder bump interconnections. The interconnections physically bond the component to the substrate and also connect the terminal pads for conveying electrical signals to and from the component for processing.
Similar solder bump interconnections have been employed to attach an integrated circuit die to a pad array carrier or directly to a printed circuit board.
The component and the substrate are typically formed of different materials. During thermal cycling such as is normally experienced by the package during operation, the component and the substrate expand and contract, but at different rates. This thermal expansion mismatch generates stresses within the solder joints that may lead to failure of the interconnection. This problem is complicated by collapse of the solder bumps during reflow. When the bump is heated to melt the solder, the weight of the component collapses the molten droplet, resulting in an interconnection having a pancake configuration wherein the midsection is large relative to the bonding area at the terminal pads. This pancake configuration concentrates thermal stress fatigue at the solder bonds to the pads. It has been proposed to stretch the solder body during reflow to form elongated interconnections that reduce the concentration of stress at the pads and thereby extend the life of the interconnection. The elongated interconnections may exhibit a pillar or preferably an hourglass configuration. Before stretching the solder, it is necessary to first permit collapse of the molten bump to assure intimate contact with the pads that is essential to form the desired solder bonds. Also, the stretching distance needs to be controlled to avoid breaking contact with the pads. Furthermore, it is desired to elongate the interconnections within the soldering furnace, without additional steps that add to production costs.